Description of the Prior Art
The present invention relates to a semiconductor device and a method of fabricating the same, and more particularly to a semiconductor integrated circuit with an improved lead frame structure and a method of forming the same.
In recent years, high density integration of semiconductor integrated circuits and large scale integrated circuits has been on the increase. Packages of the semiconductor integrated circuits and large scale integrated circuits have also been scaled down with increased number of connection pins of the semiconductor integrated circuits. For those reasons, a pitch between the adjacent leads is made narrow. This tendency of such narrowed pitch between the leads is also present when the semiconductor chip is sealed with a mold resin. Such tendency of the narrowed pitch between the leads means a high possibility of formation of a short circuit between the adjacent leads by a slight displacement of the leads. In order to settle this problem, when the semiconductor chip is mounted with a sealing epoxy resin, leads of the lead frame are fixed with tapes. This lead frame is most popular and so called as a taping lead frame.
The conventional taping lead frame structure provided by a lead frame taping method will be described with reference to FIGS. 1A and 1B. FIG. 1A is a fragmentary plane view illustrative of a conventional taping lead frame structure provided by a lead frame taping method. FIG. 1B is a fragmentary cross sectional elevation view taken along an A"-B" line of FIG. 1A and illustrative of a conventional taping lead frame structure provided by a lead frame taping method.
An insulation tape 2 is provided which is adhered with an adhesive 3 to a plurality of leads 1 so that the insulation tape 2 extends cross the plural leads. Each of the leads 1 is made of Cu. A distance between adjacent leads 1 is 0.2 millimeters. The insulation tape 2 is made of an organic insulator such as polyimide. The adhesive 3 may be made of a mixture of NBR and phenol resin.
The adhesive 3 is applied entirely on one surface of the insulation tape 2 whilst the adhesive 3 is also in contact with the plural leads 1. Namely, the adhesive 3 is formed between the insulation tape 2 and the plurality of the leads 1, and also formed on between the insulation tape 2 and gaps of the plurality of the leads 1. The adhesive 3 is disposed between the adjacent leads 3, namely the adjacent leads 1 are connected through the adhesive 3 to each other. If a potential difference or a voltage is applied across the adjacent two leads 1, then the electrolytic action causes an ionization of Cu at an anode side before ionized Cu is moved through the adhesive 3 to a cathode side where a precipitation thereof appears Namely, a Cu ion migration appears. Finally, a large amount of Cu is precipitated whereby any short circuit may be formed between the adjacent two leads 1. Even if no short circuit is formed between the adjacent two leads 1, a leakage of current between the adjacent two leads 1 is increased.
In the Japanese laid-open patent publication No. 3-284868, it is disclosed to prevent the above ion migration of a plating material at bonding portions of the semiconductor device by changing the plating material into materials other than Cu. However, the bodies of the leads 1 are made of Cu and it is difficult to prevent the Cu ion migration from the Cu bodies of the leads 1.
In the above circumstances, it had been required to develop a novel semiconductor device with an improved lead frame structure which is capable of preventing Cu ion migrations between the leads.